Diamond cuttting insert

ABSTRACT

A cutting insert, particularly a razor blade or surgical blade, including a layer of diamond having a cutting edge defined at the intersection to two converging surfaces. The converging surfaces are coated with a layer of diamond-like carbon (DLC).

BACKGROUND OF THE INVENTION

This invention relates to a diamond cutting insert and more particularlyto a diamond razor blade.

Conventional razor blades are made from steel and go blunt during thehair shaving process. Techniques to improve the longevity of steelblades include the application of hard coatings and the treatment of thesteel by, for example, ion implantation. Whilst these enhancementtechniques do work, the improvement in longevity (the length of time theblade remains sharp) is only modest.

It has been suggested that the longevity of a steel razor blade can beimproved by coating the cutting edge of the blade with a hard materialsuch as a ceramic.

It has also been recognised that harder materials than steel, such asceramics, generally make longer lasting blades. The harder the material,the longer the blade will last, provided the edge does not damage orchip.

FR2 536 691 describes a diamond razor blade made from multiple singlecrystals of diamond each of which has a sharpened edge.

U.S. Pat. No. 4,720,918 describes a steel blade having a particularprofile leading up to the cutting edge thereof. The steel blade may becoated. It is also suggested in the specification that the steel blademay be replaced by harder blade material such as sapphire, titaniumcarbide or diamond.

SUMMARY OF THE INVENTION

According to the present invention, a cutting insert, particularly arazor blade or surgical blade, comprises a layer of diamond having acutting edge defined at the intersection of two converging surfaces, theconverging surfaces, being coated with a layer, generally a thin layer,of diamond-like carbon (DLC).

The layer of DLC may also cover the cutting edge or may terminate justshort of the cutting edge. Thus, if an extremely sharp cutting edge isdesired, then the cutting edge may be provided by the diamond layer andthe DLC coating terminate on each converging surface just short of thecutting edge. Alternatively, the cutting edge may be covered by a layerof DLC. The thickness of this layer can be tailored to achieve specificobjectives. If a very sharp cutting edge is desired, then the layer canbe made extremely thin. Alternatively, if a less sharp cutting edge isdesired, then a thicker layer of DLC can be provided thereon.

The diamond layer may be monolithic in which event the cutting edge willbe continuous and uninterrupted by bonding regions. The diamond layermay also comprise a plurality of diamond pieces or segments bondedtogether. In this event, the cutting edge will have one or morediscontinuities defined by the bonding regions. When the diamond layeris monolithic, it is preferably CVD diamond.

Diamond-like carbon (DLC) (or alpha: C or alpha: C—H as it is sometimestermed) is an amorphous material, made up of carbon and hydrogen. Thestructure comprises a mixture of sp³ and sp² bonded carbon withsignificant C—H bonding and there is no long-range order. The termdiamond-like carbon arises due to its extreme hardness, e.g. Vicker'shardness as high as 6-8000 kg/mm, although this is less than that ofdiamond, generally 8-10 000 kg/mm². DLC layers are generally grown underconditions of ion bombardment and are highly stressed.

The thickness of the DLC layers will vary according to the nature of thecutting insert, but will typically be less than 1 μm, more typicallyless than 0.5 μm. An example of a thickness range is 0.1 to 0.4 μm.

The DLC coating on the converging surfaces leading to a diamond cuttingedge provides the following advantages and benefits:

-   -   1) modification of frictional properties, making it more        suitable for shaving, for example;    -   2) controlled modification of tip radius to control sharpness        and strengthen and thicken the cutting tip;    -   3) DLC is amorphous, so does not exhibit chipouts, and the        coating suppresses chipouts in the underlying diamond edges;    -   4) DLC coatings have a tendency to self planarise. Thus, the        final roughness of a coated surface can be less than the initial        surface. This is useful in coating diamond in ‘infilling’        scratches and other polishing flaws.

A DLC coating to diamond also gives rise to the following advantages:

-   -   a) exceptional adhesion because of the chemical similarity        between diamond and DLC;    -   b) diamond is well capable of supporting the intrinsic stresses        of DLC. These stresses are typically tensile, putting the        diamond under compressive stress and controlling brittle        fracture, even in single crystal diamond;    -   c) the thermal expansivity and many other properties of diamond        and DLC are much better matched than with conventional        substrates for DLC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate schematic sectional side views of twoembodiments of cutting inserts of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The cutting insert of the invention may take a variety of forms and beuseful in a variety of applications. Examples are:

-   -   a) A cutting insert or blade used in laser assisted surgery        (LAS), comprising a diamond insert and more typically a CVD        diamond insert, wherein the surfaces leading to the cutting edge        and the cutting edge are coated with a layer of DLC. CVD diamond        is diamond produced by chemical vapour deposition. In laser        assisted surgery, the laser energy is projected through the        blade to cauterise the cut tissue. In this application the DLC        coating is optimised for stable, long-term non-stick properties        for coagulated proteins from blood plasma (fibrines,        fibrinogens), tissue fluid and red blood cells. In addition the        coating provides protection during post-operative cleaning        procedures used to remove body materials adherent to the blade.        The thickness of the coating is typically 0.1-0.15 μm, providing        the bulk properties of the layer desired but avoiding        significant interference colours or optical absorption in the        layer. In this type of application, laser induced coagulation        results in the issue of material sticking to the blade being of        prime concern. Thus, for this application the DLC coating is        generally tailored to the softer end of its potential range,        with the objective of further improving the non-stick properties        of the coating, and additionally minimizing the stress in the        layer and thus maximizing its adhesion. Such layers are        generally more polymeric, and contain more H₂ or SiO.    -   b) A cutting insert used in general surgery, comprising a        diamond insert and more typically a CVD diamond insert, wherein        the surfaces leading to the cutting edge and the cutting edge        itself are coated with a layer of DLC. Again, layers are        typically 0.1 to 0.15 μm thick, providing layers free of        interference colours and with good adhesion to the diamond        substrate. The non-stick properties, although important are not        as dominant in the coating optimisation as for the laser        assisted surgery; relatively soft DLC coatings are still        generally used but these can be harder than for LAS

In application both these types of surgical blades have shown asubstantially improved incision characteristic and longer lifetimebetween cleaning operations necessary to maintain this performance,improving the utility of the blades in application.

-   -   c) A cutting insert used for human shaving, particularly for the        face, comprising a diamond insert and more typically a CVD        diamond insert, coated with a layer of DLC. The DLC coating may        be tailored to achieve particular interference colours, which        may or may not be acceptable depending on the market, and        non-stick properties to avoid the adhesion of fatty molecules        and thus minimise skin drag. The wetting properties of the DLC        coating may also be tailored to improve lubrication by        water/soap used in the shave, for example by increasing the        silica (SiO) content of the coating. Another design issue is the        sharpness of the cutting tip. The uniformity of the DLC coating        can be controlled so that the coating or layer is thinner or        absent at the very cutting tip or edge. A similar profile can        also be generated by DLC coating the blade prior to the        application of a final polishing process to the cutting edge.        Diamond can take a sharper edge than may be desirable for        shaving. The DLC layer can be tailored to increase the cutting        edge radius in a controlled manner, providing the ideal edge        sharpness in addition to an edge more resistant to chipouts.

Embodiments of cutting inserts of the invention will now be describedwith reference to FIGS. 1 and 2 of the accompanying drawings. Referringto FIG. 1, a cutting insert comprises a diamond layer 10 having acutting edge 12 defined at the intersection of two converging surfaces14, 16. Each of the converging surfaces 14, 16 is provided with a layer18 of DLC. The layers 18 terminate just short of the cutting edge 12which is thus a diamond cutting edge.

In the embodiment of FIG. 2, a layer 20 of diamond is provided having acutting edge 22 defined at the intersection of converging surfaces 24,26. The converging surfaces 24, 26 and cutting edge 22 are all coatedwith a layer 28 of DLC.

The DLC coating may be applied to the diamond cutting edge by methodsknown in the art. A wide range of parameters and methods appropriate tothe deposition of DLC coatings are known. Often the limitation on growthconditions for conventional DLC applications is placed by thesensitivity of the substrate, whereas diamond is tolerant to a muchwider range of deposition conditions, coating thicknesses, and coatingproperties.

The invention is further illustrated by the following examples:

EXAMPLE 1

A diamond blade is taken and mounted into a groove in a prepared metalholder, 3 mm thick, which holds the blade at an angle of 70° C. to thenormal of the blade, exposing one side of the cutting edge of the bladeuppermost. This metal holder is then placed onto the counter-electrodeof an RF bias physical vapour deposition system, with the metal base inintimate electrical and thermal contact with this counter electrode. Thedeposition process can be operated over a range of conditions, which canbe used to tailor the exact properties and composition of the DLCcoating produced. In general the process comprises the mixing of H₂,CH₄, and possibly an inert gas such as Ar, and maintaining this mixtureat a pressure in the range 1-500 mTorr (0.12-65 Pa) and more typically5-50 mTorr (0.66-6.6 Pa). A plasma is generated using RF frequency,typically 13.56 MHz, and an electrode bias generated by use of ablocking capacitor which is related to the relative areas of the twoelectrodes. Typical bias voltages used lie in the range 100-1000 V, andmore typically in the range 400-700 V. The substrate temperature isgenerally maintained at less than 200° C. and preferably less than 150°C. The blade is then remounted to expose the second face of the cuttingedge and the deposition process repeated.

An alternative method is the use of an ion gun, where a beam of carbonand hydrogen ions is generated and directed onto the blade in the regionof the cutting edge.

EXAMPLE 2

The outline dimensions of a number of blades were cut with a high powerlaser, out of a plate of polycrystalline CVD diamond. The blades werepolished to the appropriate thickness (0.6 mm). The technique of lasercutting diamond is well known in the field. Subsequently two facets werepolished onto each blade, the intersection of the facets forms a cuttingedge of the blade. Polishing was done by pressing the blade at an angleonto a scaife made from steel or cast iron or epoxy resin with diamondparticles embedded in the resin as is well known in the art. Other waysto produce the blades are to laser cut the facets in the plates andsubsequently polish these laser-cut facets. At this point the bladeswere shaped ready for coating.

In order to obtain a strongly adherent coating the surfaces of theblades were cleaned, to remove all dirt and and adsorbed matter (bothorganic and inorganic in nature). Cleaning was generally performed usinga concentrated acid such as nitric, sulfuric, or hydrochloric acid,followed by a rinse in demineralised water, a further rinse in analcohol such as methanol, ethanol, or propanol, and oven drying at atemperature of 60-100° C. In one particular example concentratedsulphuric acid was used followed by rinsing in water and propanol andthe blades were dried at 60° C. for 1 hour.

As a further cleaning stage, the blades were exposed to a microwaveplasma discharge which contained a high concentration of oxygenradicals. These oxygen radicals react with any remaining contaminants onthe surface of the diamond and remove them. This etch can also attackthe diamond itself, so the blade was only exposed to the plasma for ashort period of time, typically less than ½ hour. In one case the bladewas exposed to a microwave (2.45 GHz) generated plasma for 15 minutesonly and the oxygen gas pressure in the reaction chamber was 1 mbar(1×10² Pa). The discharge power was approximately 600 Watt peak powerwith a duty cycle of approximately 20%.

Following the etch, a DLC coating was deposited on the facets andcutting edges of the blades. This was done using a mixture of gasescomprising a carbon source, hydrogen, and an oxygen source to acommercially available recipe. In some cases the mixture also containedsmall amounts of a silicon source such as silane to introduce somesilica into the DLC coating. The gases where then broken down by themicrowave plasma and a layer of DLC typically about 200 nm thickness wasapplied to the facets and cutting edge of the blade. In other cases,thicker DLC coatings were applied up to approximately 4 micronsthickness although in general there is limited benefit from DLC coatingsgreater than 1 μm thickness. The Vickers hardness of the DLC coating wasapproximately 1200-1500 VH.

As fatty materials (such as lipids and other fatty materials in thehuman skin) have very weak adhesion to DLC-type layers, while on theother hand they strongly adhere to diamond, the DLC coated blades givesignificantly less drag of the skin during shaving than uncoated diamondblades. Friction tests that were done for common materials such asmetals indicate that the DLC coated diamond has a friction coefficientsignificantly lower than uncoated diamond. In one example a frictioncoefficient for the DLC coated diamond on steel was measured to be lessthan 0.05, while the uncoated diamond had friction coefficients in therange of 0.4 to 0.8.

Since the DLC coating has a diamond-like structure the adherence todiamond is exceptionally strong and this combined with inherent strengthof diamond leads to exceptional long lifetime of the coating. Theaddition of silica in some of the DLC layers was intended to reduce thestress within the layers and improve adhesion and wear characteristicseven further. In tests it was not possible to remove the DLC coatingfrom the diamond surface by mechanical means other than by (abrasive)polishing techniques such as those used for preparing the blades.Coating roughness was very low (typically this is less than 0.2 nm) thusadding little or nothing to the original roughness of the polished bladewhich was 2 nm or less, and in some instances the overall roughness wasreduced.

Control of the frictional properties of the blade with the skin does notrequire the coating to extend to the cutting edge. Such blades werefabricated. The simplest method of fabrication is to preparemechanically the final cutting edge after DLC coating by removing theDLC at the cutting edge. A particular advantage of this configuration isthat diamond wear at the precise cutting tip can be lower than that ofthe DLC coating.

However, other benefits such as suppression of chipouts are obtainableor enhanced by coating very close to or around the diamond cutting edge.The effect on the cutting geometry is dependent on the relativedimensions of the cutting tip radius and the coating thickness at thispoint, although it is also possible to shape the DLC coated cutting edgesubsequent to applying the coating.

1-13. (canceled)
 14. A cutting insert comprising: a layer of diamondhaving a cutting edge defined at an intersection of two convergingsurfaces, the converging surfaces being coated with a layer ofdiamond-like carbon (DLC).
 15. A method according to claim 14, whereinthe DLC layer also coats the cutting edge.
 16. A cutting insertaccording to claim 14 which is a razor blade.
 17. A cutting insertaccording to claim 14 which is a surgical blade.
 18. A cutting insertaccording to claim 14, wherein the layer of DLC is a thin layer.
 19. Acutting insert according to claim 18, wherein a thickness of the DLClayer is less than 1 μm.
 20. A cutting insert according to claim 19,wherein a thickness of the DLC layer is less than 0.5 μm.
 21. A cuttinginsert according to claim 20, wherein a thickness of the DLC layer is ina range of 0.1 to 0.4 μm.
 22. A cutting insert according to claim 14,wherein the diamond layer is monolithic.
 23. A cutting insert accordingto claim 14, wherein the diamond layer comprises a plurality of piecesor segments bonded together.
 24. A cutting insert according to claim 14,wherein the DLC coating contains SiO.